In semiconductor processing facilities, it is common for semiconductor wafers to be transported from semiconductor processing tool to semiconductor processing tool using a front-opening unified pod (FOUP). A FOUP typically includes a carrier housing with a vertical stack of wafer support structures, e.g., shelves that protrude from the side walls of the housing, that support a plurality of wafers in a spaced-apart stack. A typical FOUP may hold 25 or 30 wafers, although FOUPs with other capacities are possible. The wafers are typically spaced apart within the FOUP so that a wafer handling robot may insert an end effector between adjacent wafers in the stack and lift a wafer up and withdraw it without disturbing the other wafers.
FOUPs typically include a removable FOUP door that may be used to seal the FOUP from the ambient environment; a FOUP may be filled with a buffer gas of some sort that protects the wafers within the FOUP from conditions of the ambient environment within the semiconductor processing facility, e.g., from moisture or oxygen. The buffer gas may, for example, be clean dry air, nitrogen, or any other gas that is deemed more desirable than the facility air. The buffer gas is not, however, the same as the facility air that is typically flowed through the EFEM.